This invention relates to a novel process of producing chirally pure xcex2-amino-alcohols, as well as intermediates thereof including xcex1-amino acids. Compounds of the present invention are useful for a variety of purposes, including for use in pharmaceutical compositions.
A variety of techniques have been described for production of a preferred enantiomer from xcex1-amino acids. These techniques require the use of either resolution procedures or asymmetric syntheses at some point in the synthesis to prepare the target compounds. More efficient means for producing chirally pure target compounds are needed.
In one aspect, the present invention comprises a process for preparing chirally pure S-enantiomers of xcex1-amino acids.
In a further aspect, a process is provided for preparing chirally pure S-enantiomers of xcex2-amino alcohols.
In yet another aspect, a process is provided for preparing chirally pure S-enantiomers of N-sulfonyl xcex2-amino alcohols.
These and other aspects of the invention will be apparent to one of skill in the art upon reading of the following detailed description of the invention.
The present invention is directed to a process for the preparation of chiral xcex1-amino acids of the formula (R)2CH(CH2)nCH(NH2)C(xe2x95x90O)OH, where n is 0 to about 10; chiral xcex2-amino alcohols of the formula (R)2CH(CH2)nCH(NH2)CH2OH; and chiral S enantiomers of N-sulfonyl xcex2-amino alcohols of the formula (R)2CH(CH2)nCH(CH2OH)NHxe2x80x94S(O)2-2-C4H2S-5-Cl, wherein R is lower alkyl, substituted lower alkyl, lower alkenyl, substituted lower alkenyl, lower alkynyl, substituted lower alkynyl, cycloalkyl, substituted cycloalkyl, phenyl, substituted phenyl, benzyl, substituted benzyl, CH2cycloalkyl, CH2-3-indole, CH(loweralkyl)-2-furan, CH(loweralkyl)-4-methoxyphenyl, CH(loweralkyl)phenyl, or CH(OH)-4-SCH3-phenyl.
Both natural and unnatural xcex1-amino acids, natural and unnatural xcex2-amino alcohols, and intermediates thereof, may be prepared according to the present invention. Such xcex1-amino acids and xcex2-amino alcohols may also be referred to as 2-amino acids or 2-amino alcohols.
As used herein, the term xe2x80x9cchirally purexe2x80x9d refers to compounds which are in 100% S-enantiomeric form as measured by chiral high performance liquid chromatography (HPLC). Other methods of measuring chiral purity include conventional analytical methods, including specific rotation, and conventional chemical methods. However, the technique used to measure chiral purity is not a limitation on the present invention.
As described herein, the method of the invention affords chirally pure xcex1-amino acids or xcex2-amino alcohols following the recrystallization step in the method. Where chiral purity is not a requirement, the method of the invention may also be used to provide chiral xcex1-amino acids or xcex2-amino alcohols which contain some percentage of a mixture of enantiomeric forms, e.g., which may be composed of about 90 to about 99% S-enantiomers, by following the method of the invention in the absence of recrystallization.
In one embodiment, the present invention is directed toward a process for preparing chiral S-enantiomers of xcex1-amino acids, which involves preparing an organometallic reagent from an alkyl halide of the formula (R)2CH(CH2)nCH2X, wherein X is Cl, Br or I and n is 0 to about 10; adding the organometallic reagent to carbon dioxide to afford a carboxylic acid; activating the carboxylic acid with an acid halide, phosphorus trichloride, acid anhydride, or thionyl chloride in the presence of a tertiary amine base; reacting the product of the activating step with an alkali metal salt of S-4-benzyl-2-oxazolidinone; treating the product of the alkali metal step with a strong non-nucleophilic base to form an enolate anion; trapping the enolate anion with 2,4,6-triisopropylbenzenesulfonyl azide to afford an oxazolidinone azide; hydrolyzing the oxazolidinone azide with an aqueous base to afford an xcex1-azido acid; reducing the xcex1-azido acid to the xcex1-amino acid; and recrystallizing the xcex1-amino acid to afford the chirally pure xcex1-amino acid.
In another embodiment, the present invention is directed toward a process for preparing chiral S enantiomers of xcex2-amino alcohols, which involves preparing an xcex1-amino acid as described above, reducing the xcex1-amino acid to the xcex2-amino alcohol, and recrystallizing the xcex2-amino alcohol to afford the chirally pure xcex2-amino alcohol.
In a further preferred embodiment, the present invention is directed toward a process for preparing chiral S enantiomers of N-sulfonyl xcex2-amino alcohols of the general formula: 
wherein R is lower alkyl, substituted lower alkyl, lower alkenyl, substituted lower alkenyl, lower alkynyl, substituted lower alkynyl, cycloalkyl, substituted cycloalkyl, phenyl, substituted phenyl, benzyl, substituted benzyl, CH2cycloalkyl, CH2-3-indole, CH(loweralkyl)-2-furan, CH(loweralkyl)-4-methoxyphenyl, CH(loweralkyl)phenyl, or CH(OH)-4-SCH3-phenyl and n is 0 to about 10. This process involves reducing an xcex1-amino acid to an xcex2-amino alcohol of the formula (R)2CH(CH2)nCH(NH2)CH2OH; sulfonylating the xcex2-amino alcohol with 5-chloro-thiophene-2-sulfonyl halide; and recrystallizing the product of the sulfonylation step to afford the chirally pure N-sulfonyl xcex2-amino alcohols.
In one embodiment, the compounds of the invention contain one chiral carbon center, where R in the above-noted structures is the same. In certain desired embodiments, the R groups are methyl, ethyl, and n-propyl, and most preferably the R groups are ethyl. However, the invention further encompasses producing xcex1-amino acids and xcex2-amino alcohols of the general formulae provided herein where the R groups are different. In these compounds one or more additional chiral centers may be present; however, the additional chiral centers must be optically pure and must not interfere with the production of the chirally pure xcex1-amino acids, xcex2-amino alcohols, and pure S enantiomers of N-sulfonyl xcex2-amino alcohols of the present invention.
In another preferred embodiment, the chiral carbon center is of S-stereochemistry which gives rise to enantiomerically pure products.
Thus, the process of the invention provides an efficient route to the synthesis of chirally pure S enantiomers of xcex2-amino alcohols, and intermediates thereof, which are useful for a variety of purposes. For example, the exemplary compounds provided herein, the N-sulfonyl xcex2-amino alcohols are useful for inhibition of xcex2-amyloid production, which is implicated in amyloid angiopathy, cerebral amyloid angiopathy, systemic amyloidosis, Alzheimer""s Disease (AD), hereditary cerebral hemorrhage with amyloidosis of the Dutch type, inclusion body myositis, Down""s syndrome, among others.
As used herein, the term xe2x80x9cpharmaceutically usefulxe2x80x9d refers to compounds having a desired biological effect, whether as a therapeutic, immune stimulant or suppressant, adjuvant, or vaccinal agent. Similarly, a variety of compounds which are suitable for use in non-pharmaceutical applications, e.g., a diagnostic, a marker, among others may be produced by the method of the invention. However, other pharmaceutically useful compounds may be produced by this method.
The term xe2x80x9calkylxe2x80x9d is used herein to refer to both straight- and branched-chain saturated aliphatic hydrocarbon groups having one to ten carbon atoms, preferably one to eight carbon atoms and, most preferably, one to six carbon atoms; xe2x80x9calkenylxe2x80x9d is intended to include both straight- and branched-chain alkyl groups with at least one carbonxe2x80x94carbon double bond and two to eight carbon atoms, preferably two to six carbon atoms; and xe2x80x9calkynylxe2x80x9d group is intended to cover both straight- and branched-chain alkyl groups with at least one carbonxe2x80x94carbon triple bond and two to eight carbon atoms, preferably two to six carbon atoms. As used herein, the term xe2x80x9clowerxe2x80x9d refers to any of the above-defined groups having one to six carbon atoms.
The terms xe2x80x9csubstituted alkylxe2x80x9d, xe2x80x9csubstituted alkenylxe2x80x9d, xe2x80x9csubstituted alkynylxe2x80x9d, xe2x80x9csubstituted lower alkylxe2x80x9d, xe2x80x9csubstituted lower alkenylxe2x80x9d, and xe2x80x9csubstituted lower alkynylxe2x80x9d refer to alkyl, alkenyl, alkynyl, lower alkyl, lower alkenyl, and lower alkynyl as just described having from one to three substituents which are independently selected from among halogen, CN, OH, NO2, amino, aryl, heterocyclic, substituted aryl, substituted heterocyclic, alkoxy, substituted alkoxy, aryloxy, substituted aryloxy, alkylcarbonyl, alkylcarboxy, alkylamino, and arylthio. These substituents may be attached to any carbon of an alkyl, alkenyl, or alkynyl group provided that the attachment constitutes a stable chemical moiety.
The term xe2x80x9csubstituted phenylxe2x80x9d refers to a phenyl group having one to four substituents which are independently selected from among halogen, CN, OH, NO2, amino, alkyl, cycloalkyl, alkenyl, alkynyl, alkoxy, aryloxy, substituted alkyloxy, alkylcarbonyl, alkylcarboxy, alkylamino, and arylthio.
The term xe2x80x9ccycloalkylxe2x80x9d refers to a carbon-based ring having more than 3 carbon atoms contained in the backbone of the ring.
The term xe2x80x9csubstituted benzylxe2x80x9d refers to a benzyl group having one to four substituents which are independently selected from among halogen, CN, OH, NO2, amino, alkyl, cycloalkyl, alkenyl, alkynyl, alkoxy, aryloxy, substituted alkyloxy, alkylcarbonyl, alkylcarboxy, alkylamino, and arylthio.
The term xe2x80x9chalogenxe2x80x9d refers to chlorine, bromine, fluorine, or iodine.
The term xe2x80x9cstrong non-nucleophilic basexe2x80x9d refers to a non-nucleophilic basic reagent, which does not act as a nucleophile or bind to the reagents utilized according to the reaction. A number of non-nucleophilic bases are known in the art and include sodium hydride, potassium hydride, lithium diisopropylamide and potassium hexamethyldisilazide.
The term xe2x80x9caqueous basexe2x80x9d refers to a solution composed of at a minimum a base and water. A number of bases which readily dissolve in water are known in the art and include alkali metal hydroxides such as lithium hydroxide, sodium hydroxide or potassium hydroxide, among others. The aqueous base solution may further contain other reagents which do not interfere with the reactions of the present invention, and include organic solvents such as tetrahydrofuran, methanol, or hydrocarbon solvents, salts such as sodium chloride, and buffers, among others.
The term xe2x80x9corganic solventxe2x80x9d may include any solvent known in the art, which does not react with the reagents utilized in the reaction and includes saturated hydrocarbon solvents, unsaturated hydrocarbon solvents, including aromatic hydrocarbon solvents, alcohols, halocarbons, ethers, and acetates, among others.
The compounds of the present invention can be used in the form of salts, e.g., derived from pharmaceutically or physiologically acceptable acids or bases. These salts include, but are not limited to, the following salts with organic and inorganic acids such as acetic, lactic, citric, tartaric, succinic, fumaric, maleic, malonic, mandelic, mallic, hydrochloric, hydrobromic, phosphoric, nitric, sulfuric, methanesulfonic, toluenesulfonic and similarly known acceptable acids, and mixtures thereof. Other salts include salts with alkali metals or alkaline earth metals, such as sodium (e.g., sodium hydroxide), potassium (e.g., potassium hydroxide), lithium, calcium or magnesium.
These salts, as well as other compounds of the invention may be in the form of esters, carbamates and other conventional xe2x80x9cpro-drugxe2x80x9d forms, which, when administered in such form, convert to the active moiety in vivo. In a currently preferred embodiment, the prodrugs are esters. See, e.g., B. Testa and J. Caldwell, xe2x80x9cProdrugs Revisited: The xe2x80x9cAd Hocxe2x80x9d Approach as a Complement to Ligand Designxe2x80x9d, Medicinal Research Reviews, 16(3):233-241, ed., John Wiley and Sons (1996).
In one embodiment, the xcex1-amino acids and xcex2-amino alcohols of the invention are reacted with a variety of reagents to form complexes having at least one chiral carbon center. In one embodiment, the xcex1-amino acids and alcohols are reacted with thiophene sulfonyl halides, more desirably, 5-halo thiophene sulfonyl halides, and most desirably, 5-chloro-thiophene sulfonyl halides to form the chirally pure heterocyclic N-sulfonyl xcex2-amino-alcohols of formula (8).
In another embodiment, the xcex1-amino acids or xcex2-amino alcohols of the invention are reacted with furansulfonyl halides to form chirally pure heterocyclic N-sulfonyl xcex2-amino-alcohols.
The following scheme (Scheme 1) will facilitate further a general understanding of the invention by those skilled in the art, while Scheme 2 describes a preferred embodiment of the instant invention. Those skilled in the art will readily understand how to apply the process of this invention to the various embodiments encompassed by this invention. 
Referring to Scheme 1, conversion of the alkyl halide (1) to the carboxylic acid (2) may be achieved by initial conversion of the alkyl halide (1) to an organometallic reagent. Various techniques are known in the art to convert alkyl halides to organometallic reagents. See, e.g., Organometallic Syntheses, Volume 2, John J. Eisch, ed., Academic Press, New York, 1981. Preferably, the alkyl halide is an alkyl bromide, chloride, or iodide. More preferably, the alkyl halide is an alkyl bromide. A variety of metals and organometallic reagents are known to facilitate conversion of alkyl halides to carboxylic acids and include Grignard reagents, organolithium reagents, magnesium and lithium metals, among others. Once prepared, the organometallic reagent is converted to a carboxylic acid, preferably by quenching with carbon dioxide. Alternatively, conversion to the carboxylic acid may be by any other suitable method known in the art. Such methods include reacting the organometallic reagent with diethyl carbonate or ethyl chloroformate to afford the ethyl ester, which is hydrolyzed to the carboxylic acid using an aqueous base. A variety of aqueous bases may be selected by one of skill in the art, and include alkali metal hydroxides such as lithium hydroxide, sodium hydroxide or potassium hydroxide, among others.
The carboxylic acid (2) is then converted to the oxazolidinone derivative (3). The carboxylic acid (2) is first converted to an activated carbonyl species by reaction of the carboxylic acid with a reagent, including, but not limited to, acid halides, phosphorus trichloride, acid anhydrides, or thionyl chloride, followed by reaction with a tertiary amine. A variety of acid halides may be utilized and include acid chlorides, bromides, and iodides. Preferably, the acid halide is an acid chloride. A variety of acid chlorides are known in the art and include pivaloyl chloride, isovaleryl chloride, ethyl chlorocarbonate, and isobutyl chlorocarbonate, among others. Most preferably, the acid chloride is pivaloyl chloride. A variety of acid anhydrides are known in the art and include trifluoroacetic anhydride and trichloroacetic anhydride. Preferably, the acid anhydride is trifluoroacetic anhydride. A number of tertiary amines are known in the art and include triethylamine, trimethyl amine, N,N-diisopropylethyl amine, and pyridine, among others. Preferably, the tertiary amine is triethylamine.
The activated carbonyl species is subsequently reacted with an alkali metal salt of a chiral auxiliary reagent in a suitable organic solvent. Preferably, the chiral auxiliary agent is S-4-benzyl-2-oxazolidinone. However, other chiral auxiliaries may be utilized and readily selected by one of skill in the art. See, e.g., Principles and Applications of Asymmetric Synthesis, G. Lin, Y. Li, and A. Chan, Wiley-Interscience, New York, 2001 (for example, page 104, Tables 2-13). Alkali metal salts of S-4-benzyl-2-oxazolidinone which are useful in this reaction include lithium, sodium, and potassium salts. Preferably, the chiral auxiliary is the lithium salt of S-4-benzyl-2-oxazolidinone.
In an effort to maximize product yield, conversion of the acid to the oxazolidinone derivative is preferably performed in about 30 minutes. However, the reaction time may be dependent upon a variety of factors including reaction temperature, purity of the reagents, scale of the reaction, environmental conditions, exact structure of the substrate, and concentration, among others. Longer or shorter reaction times (e.g., 10 to about 60 minutes) may be utilized as determined by one of skill in the art. 
In a preferred embodiment, the carboxylic acid (2) is converted to the mixed anhydride by reaction with pivaloyl chloride in the presence of triethylamine and subsequently reacted with the lithium salt of S-4-benzyl-2-oxazolidinone (generated by the action of n-butyllithium on S-4-benzyl-2-oxazolidinone) in tetrahydrofuran (THF) to form the oxazolidinone derivative (3). See, Scheme 2.
The oxazolidinone derivative (3) is then converted to its enolate anion by the action of a strong non-nucleophilic base, as defined herein. Preferably, the strong non-nucleophilic base is potassium hexamethyldisilazide. The enolate anion is then reacted with trisyl azide to form the azido-oxazolidinone intermediate (4).
The azido-oxazolidinone intermediate (4) can be converted to the xcex1-azido-acid (5) by any suitable method known in the art. In a preferred embodiment, the azido-oxazolidinone intermediate is converted to the xcex1-azido acid by hydrolysis with an aqueous base, as defined herein. Preferably, the azido-oxazolidinone intermediate is converted to the xcex1-azido acid by hydrolysis using an aqueous solution of lithium hydroxide.
The xcex1-azido-acid (5) can be reduced to the xcex1-amino-acid (6) by any suitable method known in the art. Preferably, the reduction is performed using catalytic reduction with hydrogen gas in the presence of 10% palladium on carbon catalyst. Alternatively, the reduction may be performed with zinc/HCl, sodium borohydride, or aqueous triphenyl phosphine. In an effort to maximize product yield, the reduction is desirably performed in about 24 hours. However, the reaction time may be dependent upon a variety of factors including reaction temperature, purity of the reagents, scale of the reaction, environmental conditions, exact structure of the substrate, and concentration, among others. Longer or shorter reaction times (e.g., about 12 hours to about 96 hours) may be utilized as determined by one of skill in the art.
The chiral xcex1-amino acid may then be isolated using techniques known by those of skill in the art including, but not limited to, chromatography and recrystallization. Recrystallization may be performed using a variety of organic and inorganic solvents known in the art and provides chirally pure xcex1-amino acids.
Alternatively, the xcex1-amino acid (6) can be reduced to the xcex2-amino alcohol (7) by a variety of methods known in the art. In a preferred embodiment, reduction of the xcex1-amino acid is accomplished with catalytic hydrogenation, diborane, related boranes such as catecholborane, lithium borohydride/trimethyl silyl chloride (TMSCl), lithium aluminum hydride, diisobutyl aluminum hydride (DiBAL-H), bis(2-methoxyethoxy) aluminum hydride (Red-Al), and alane. More preferably, the reduction is accomplished using lithium borohydride/TMSCl over 48 hours.
The chiral xcex2-amino alcohol may then be isolated using techniques known by those of skill in the art including, but not limited to, chromatography and recrystallization. Recrystallization may be performed using a variety of organic and inorganic solvents known in the art.
Alternatively, the xcex2-amino-alcohol (7) is converted to the target chiral compound (8) by reaction with 5-chloro-thiophene-2-sulfonyl chloride in the presence of a strong non-nucleophilic base such as a tertiary amine or alkali metal hydroxide. Recrystallization using an appropriate solvent system using techniques known in the art affords the chirally pure target compound.
The concise nature of the reaction sequence, ease of synthesis, scalability and abundance of potential starting materials makes this process a practical method for the preparation of chirally pure S enantiomers of N-sulfonyl xcex2-amino alcohols.
Where catalysts or solvents are included in a reaction step of this invention, it is expected that other catalysts or solvents known in the art, but not mentioned herein, may be used; those skilled in the art will readily be able to determine suitable catalysts, solvents and reaction conditions for each reaction step included in the invention.
The invention includes certain types of reactions, such as enolate trapping, hydrolysis and reduction reactions, that are generally known in the art, but previously had not been applied in the novel manner of the present invention. Variations in the specific methods of accomplishing individual steps of the invention may be apparent to those in the art. Although all these possible variations cannot be set forth herein, such variations are contemplated to be within the scope of the present invention.
The following examples are provided to illustrate the production and activity of representative compounds of the invention and to illustrate their performance in a screening assay. One skilled in the art will appreciate that although specific reagents and conditions are outlined in the following examples, these reagents and conditions are not a limitation on the present invention.